Device for automatic frequency control, more particularly for the reception of carrier shift signals



1954 A. VIOLET ET AL 2,668,871

DEVICE FOR AUTOMATIC FREQUENCY CONTROL, MORE PARTICULARLY FOR THE RECEPTION OF CARRIER SHIFT SIGNALS 2 Sheets-Sheet 1 Filed May 25, 1951 ATTORNEY Fly. 2

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n n N Illl m 1| 4 A. VIOLET ET AL 2,668,871 DEVICE FOR AUTOMATIC FREQUENCY CONTROL, MORE PARTICULARLY FOR THE REC HIFT SIGNALS EPTION OF CARRIER S Filed May 25, 1951 2 Sheets-Sheet 2 INVENTOR ANDRE VIOLET MARCEL CORN/(LEAH A TTOR/VEV Patented Feb. 9, 1954 DEVICE FOR AUTOMATIC FREQUENCY CON- TROL, MORE PARTICULARLY FOR THE RECEPTION OF CARRIER SHIFT SIGNALS Andr Violet and Marcel Cornilleau, Boulogne- Billancourt, France, assignors to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application May 25, 1951, Serial No. 228,204

4 Claims. 1

The present invention concerns improvements in devices for automatic frequency control and particularly for the reception of signals transmitted by changes in frequency such as by the so called carrier shift frequency method.

In such systems, transmission is made by utilising a predetermined central frequency and modulating central frequency by a signal, which can take two values; e. g., by increasing and decreasing the said central frequency by a predetermined quantity. One frequency thus determined is generally designated in the art as space frequency and the other frequency is designated as mark frequency. In this type of system it is necessary that the tuning frequency of the receiving device coincides with the mean frequency from the transmitting device. Thus it is desirable to provide, in the receiving device an automatic frequency control system; especially since the two frequencies can vary independently. When the transmission is stopped for a certain time, for example on the space frequency, the automatic frequency regulating device tunes the receiving device on the space frequency. Under these conditions, when the transmission starts again, the receiving device is detuned and during the time interval which elapses between the start of the transmission and the moment at which the automatic frequency regulating device is operated, false signals result at the output of the receiving device.

One of the objects of the present invention is to provide an automatic frequency regulating device, which, without manipulation between two transmissions of messages, maintains the frequency of the local oscillator at a value such that the mean frequency of the received signal coincides continuously with the tuning frequency of the receiver,

According to one aspect of the invention, an automatic frequency regulating device applicable in particular to receiving devices of frequency modulated telegraphic signals, comprises, in combination, means for deriving from received signals, signals of constant amplitude and duration, means for detecting the presence or ab sence of signals and controlling in the absence of signals, the frequency of the receiving oscillator at a predetermined value with respect to the mean frequency of the signal on which the receiving device remains blocked.

According to another aspect of the invention, the means for detecting the presence or the absence of the signals comprises, in combination, a detector circuit, means for applying thesau- 2 stant amplitude signals to said detector circuit, the output from said detector device for blocking an electron tube in presence of signals and to render it conductive in the absence of signals. and means for utilising the output of said electron tube for controlling the automatic regulating frequency device.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood, by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

Figure 1 illustrates a well known system of automatic frequency control;

Figure 2 illustrates graphically the output of the discriminator; and

Figure 3 illustrates an automatic frequency control system incorporating the features of the invention.

In the description, the central frequency of the transmitter will be designated by f0; fo-i-N being the mark frequency and fo-N being the space frequency. It is assumed that when the received frequency corresponds to the mark frequency, i. e. f0+N, the potential of terminal 2 is higher than the potential of terminal 3 (Fig. 1) and that when the received frequency is the space frequency, i. e. fo-N, the potential of terminal 2 is lower than the potential of terminal 3. When the tuning frequency of the receiving system is equal to the central frequency fo, the signals resulting at the output of circuit I have a constant amplitude, their sign being a function of the frequency of the current received, fo+N or fo-N. As the number of signals corresponding to mark frequency and space frequency vary according to the character transmitted. it is impossible to utilise the average of the voltages obtained on terminals 2 and 3 as the control voltage for the regulating device of the frequency of the local transmitter. The positive and negative impulses are rectified by the rectifiers 4 and 5 respectively. The impulses which pass through rectifier 5 charge condenser 6; the discharge current of this condenser passes through resistances 1 and i3 producing a certain voltage at point 9. In

a similar way, the positive impulses which pass through rectifier 4 charges condenser l0, and the discharge current 'of condenser 10 passes through resistances H and I2, producing a certain voltage at point I3. If the local oscillator is correctly adjusted so as to make coincident the tuning frequency of the receiver with the trio es 2 and 26 and associated elements.

mean frequency fo, the crest voltages of the impulses which pass through rectifiers 4 and 5 are equal, and no current will pass through resistances I4 and I5. Point I6 is then at the same potential as that of point N. If the frequency, either of the oscillator or of the transmitter has varied, the crest voltages of the impulses passing through rectifiers 4 and 5 are different and a current passes through resistances I 4 and I5, and a difference of potential will appear between points I6 and I L The sense of this difference of potential is determined by the sense of the variation between the frequency of the transmitter and the frequency 'of the local oscillator. to point I'I, triode I8 is cut off and the potential on the grid of triode I9 increases in relation to the potential of the cathode, and this triode becomes conductive. The anode current passes through the winding of relay which closes, by its make contact, the energisation circuit of a motor ZI which controls the variable condenser represented schematically as 22 which may be coupled to the local oscillator circuit or to a control circuit for the local oscillator, in a well known manner, so as to bring back the frequency of the local oscillator and to compensate for the drift. If point I6 is negative in relation to point [1, triode I8 becomes conductive whereas triode I9 is blocked. The anodic current of triode 18 causes the operation of relay 23 which closes by its make contact, the operating circuit of motor 24 which drives variable condenser 22 in an opposite direction so as to compensate for the drift. Both motors 2i and 24 can evidently be replaced by a single motor the direction of movement of which is controlled by contacts of relay 20 and 2I. If between transmissions of signals the transmitting device remains blocked onto a frequency different from the mean frequency, for example on the space frequency, there appears a difference of potential between points 9 and I3 and thus between points I6 and I I, and the frequency regulating device has a tendency to bring back the frequency of the local oscillator so as to tune the receiving device on space fre uency. However, this presents'certain drawbacks: in particular, in the absence of manipulation, the impulsive voltages due to radioelectric noises can ive rise to false signals which trigger relavs of the teleprinter. On the other hand,

when after a rather long period durin which the transmittin device has remained blocked on s ace freouency or mark fre uency) a' certain lapse of time is necessary after resuming the transmission for regulating the fre uenc of the oscillators. Errors can occur during this period.

Referring now to Fi 3. there is shown a system which in accordan e with the invention, causes the freouency of the receiving device to automat cally-return to a frequencv corres ondins to the central fre uency. The elements of Fig. 3 hi h have the same function as the corresponding elements in Fig. 1 have been indicated b the same reference numerals. A portion of the output current from the discriminator circuit I is applied to the re ulatin circuit comprising rectifiers 4 and 5, condensers 6 and I 0, and

resistances l. 8, II and I2 as explained in relation to Figure 1. The other portion of output current from the d scr minator I is applied to the input terminals of fl p fion circuit comprisin The flip-flop circuit which has two; equilibrium If point IG-ispositive in' relation .a transformer 33.

32 which can be a vacuum tube or a gas tube.

Tube 32 is fed alternating current by means of In the absence of manipulation signalsptube. 32 is conductive during one half cycle of the alternating voltage transmitted to it by means of transformer 33. The direct current passes through resistances 34 and-35 while the alternating component of the current is by-passed by condenser 36. A voltage-appears between points 31 and 38 and :be-

tween points 38 and 39; if resistances and :35 are equal, the differences of potential between 31 and 38, and 38 and 39 are equal. By means of change over switch 4!, the one of thesetwo voltages which has a suitable polarity, either to point 42 or to point 43, is applied according to therelative setting of mark frequency and space frequency of the transmitter. According to the position of switches 40 and M the potential at point 31 or point 39 is applied either to point 42 or to point 43. If it is assumed that switches 40 and II are in the position represented on Figure 3 a current passes through resistances II and I2 and the potential of point I6 ismaintained at a predetermined value in relation to the potential corresponding to the continuous reception of space frequency. When the manipulationqis resumed, tube 32 is blocked and the regulating voltage disappears.

As it is assumed that the frequency shift remains constant for a given transmitter, the signal and value of the regulating voltage taken on terminals of potentiometer M are determined by the amplitude of the frequency shift in relation to the mean frequency and by the frequency on which the transmitter remains blockedin the absence of manipulation. If in'the absence of manipulation the frequency of the transmitter varies, the regulating device operates and makes coincident the tuning frequency of the receiving device with the mean frequency of the transmitter, thus avoiding false signals and errors which would otherwise occur at the start of the reception of the new signals.

' While the principles of the invention have'been described above in connection with specific embodiments, and particular modifications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

What is claimed is:

1. In a frequency shift signal system wherein intelligence is transmitted by alternatepulses of two different frequencies lying on opposite sides of a central frequency, and having a receiver comprising, a regulating circuit for deriving voltages of opposite polarity in response to receipt of said respective frequencies, a local oscillator coupled to the output of said regulating circuit and tunable in opposite directions toward said central frequency in response to the sum of said voltages, a circuit for correcting the tuning .ofxsaidoscillator for departures from-said central frequency due to predominance of one of said frequencies comprising means having an output circuit coupled to said regulating circuit and responsive to receipt of continued pulses of said one frequency for producing an output volt age of a predetermined magnitude, and means for applying said output voltage to said regulating circuit in opposed polarity to said derived voltage produced by said one frequency.

2. The system according to claim 1, wherein the two difierent frequencies correspond to the mark and space telegraph signals respectively, said regulating circuit comprising a first circuit operative in response to said mark signals for producing a first voltage of given polarity, a second circuit operative in response to said space signals for producing a second voltage of oppodominance of one of said frequencies for rendermg said tube conducting.

4. The system according to claim wherein said electron tube comprises cathode, control grid and anode electrodes, an integrator circuit for receiving said pulses coupled in the cathodegrid circuit of said tube, means for normally biasing said tube to cut-off, said integrator circuit producing a voltage sufiicient to overcome said bias upon receiving said predominant frequency for a given time, said given time being determined by the time constant of said integrator circuit.

ANDRE: VIOLET. MARCEL CORNILHJ'AU.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,495,326 Hansell Jan. 24, 1940 2,477,962 Chapin Aug. 2, 1949 2,477,963 Chapin Aug. 2, 1949 2,541,128 Violet Feb. 13, 1951 

